Stabilization of liquid sulfur trioxide and oleums



fates Unite STABILIZATION OF LIQUID SULFUR TRIOXIDE AND OLEUMS No Drawing. Application January 3, 1956 Serial No. 556,816

8 Claims. (Cl. 23-167) This invention relates to liquid sulfur trioxide and high strength oleums that are stabilized against polymerization; and to the stabilization thereof employing titanium and/or silicon dioxides as polymerization inhibitors.

As is Well known, sulfur trioxide exists in three forms having melting points of 17 C., 32.5 C., and 62 C. The polymeric form melting at 62 C., alpha S is the stable form under ordinary conditions. Upon freezing or even after standing at room temperature a short time the liquid gamma form (M. F. 17 C.) ordinarily changes to the solid forms.

This transition of liquid sulfur trioxide to solid polymeric forms is extremely undesirable from the standpoint of ease in handling and usage. In most instances, prior to usage the solidified sulfur trioxide must be remelted by heating to temperatures up to about 100 C., thereby developing dangerous high pressures in some instances.

High strength oleums of free sulfur trioxide strengths upward of about 80% tend to polymerize in a similar fashion. The degree of polymerization depends chiefly upon the sulfur trioxide strength of the oleum and the temperautres at which the material is stored. While poly merization will not proceed sufliciently far in some of these oleums to give a completely solid product, the degree of polymerization increases on the passage of time so that the ultimate mass will ordinarily contain solid forms of 80;, in suspension in the oleum. This polymerization of sulfur trioxide in high strength oleums is also highly undesirable.

I have discovered that liquid sulfur trioxide and oleum of 80;, strength such that S0 polymers tend to form can be stabilized against S0 polymerization by incorporating therein a minor amount of titanium and/or silicon dioxides, and then heating the product for several hours in the range of about 50 C.-l00 C. The preferred heat treatment is for at least 6 hours at about 80 C.

Titanium dioxide and silicon dioxide can be used as such; also they can be used in an activated form which has dehydrating characteristics, such as silica gel. Alternatively, the source of titanium or silicon dioxide can be a complex metal oxide containing SiO or TiO as part of the complex. For example, one or more silicates, such as a calcium aluminum silicate or sodium aluminum silicate, and oxidic titanates, such as alkali metal titanates, can be used.

The amount of these inhibiting agents to be incorporated in the S0 or oleum will vary with the use for which the SO -containing product is intended and with the conditions it will be expected to meet. The stabilizing influence of these inhibiting agents is proportional to the amount present. In general, it usually is desirable to use only sufficient amount to stabilize the product against substantial polymerization of S0 although relatively larger amounts of these inhibiting agents can be used where the presence of an excess is not objectionable. In most cases it is desirable to use no more than about 10% combined weight of titania and/or silica with from 0.2 to 5% based on the free 80, content being the preferred "ice 2 range. The higher percentages ordinarily are employed t stabilize the oleums.

The preferred compositions of this invention consist c liquid sulfur trioxide or oleum containing not more tha about 0.1% of water, which materials are stabilized wit from 0.2 to 5% by weight of titania and/ or silica. Oleur containing not more than about 0.1% of water is, c course, oleum of S0 strength not less than about 99.49 and sulfuric acid content of not more than 0.56%. Thes materials high in sulfur trioxide can be stabilized readil so as to resist polymerization even after storage for Ion periods of time.

The stabilizing agent can be added directly to th liquid sulfur trioxide or oleum to be stabilized followe by thorough agitation of the liquid. Alternatively, i the case of stabilizing pure liquid sulfur trioxide, SC vapor can be condensed in a closed vessel containing th desired amount of stabilizing agent.

Materials stabilized in accordance with my method d not polymerize to any appreciable extent. They remai substantially in the liquid gamma form at room temper: ture. Furthermore, the stabilization inhibitor functior as an antifreeze, lowering the freezing point substantiall in most instances. After freezing by being cooled to a lo temperature, these stabilized materials melt rapidly upo being reheated to a temperature of about 40 C. or les: Therefore, these products can be used directly in con mercial application without first putting them through tedious remelting process.

In order that the invention can be better understoor the following examples in addition to those set fort above are given:

Example 1 Two percent by weight of silicon dioxide in the form c silica gel is added to anhydrous liquid gamma sulfur tr oxide. The mixture is then heated for 16 hours at a ten perature of C. Upon standing at room temperatur for several days, the thusly stabilized sulfur trioxide show no tendency toward polymerization and solidification.

This mixture is found to have a freezing point of 5 C demonstrating the antifreeze action of this polymerize tion inhibitor. Upon warming to 35 C., the sulfur tr oxide mixture melts.

Example 2 One percent by weight of a sodium aluminum silicat marketed under the proprietary name Molecular Sieve by the Linde Air Company is added to liquid anhydrot gamma sulfur trioxide with agitation. This mixture then heated for 16 hours at a temperature of 70 After standing at room temperature for several days, th thusly stabilized sulfur trioxide exhibits no tendency tr ward solidification. This mixture has a freezing point 5 C. and remelts on warming to 35 C.

Example 3 Five percent by weight of finely divided anorthite, calcium aluminum silicate mineral, is added to oleui of 99.5% of S0 strength with agitation. The mixtui is then heated to C. for 10 hours. Upon standin at room temperature for several days, the thusly stabilize oleum shows no tendency toward polymerization an solidification.

Example 4 One percent by weight of titania of the rutile type added with agitation to anhydrous liquid sulfur trioxid The mixture is then heated for 24 hours at a temperatul of 80 C. After standing at room temperature for sever: days, the thusly stabilized sulfur trioxide shows no tent ency toward polymerization and solidification. This mt ture has a freezing point of 5 C. and remelts on warmir to 35 C.

.tQleumfiand liquid rioxide arealsoaestabilizr diin accordance with the procedures of the above examples,

ofa material selected fromrthe group: consistingtaofi silica andztitaniaandythen heatingpfor'several hours at a.=temperature in the range of 50 C. to 100 C.

. ZT-hemethoddfstabilizing liquid sul'furtrioxidetcontaining not-more than '0. 1% of; -'water..against:=SO polymerization -comprising incorporating :itherein about 10.2 to 5% .by Weight.based.onzSo contentcf: a material selected-from the group. consisting-mt lsilica .andfitania.

3. The method .of.:stab ilizing liquid sulfur :trioxide against S polymerization comprising incorporating therein about 0.2.to by weight'basedeonisO content 05a material selected from the groupiconsistingsof silica and-titania.

"4 A. composition containing a .material selected from the group consisting of sulphur :trioxide .andbleurn of S0 strengtl1;such that SO polymers tendto form, and a material selected from the group consistingof silica and titania; in an amount up to byweight based on S0 content.

5. A composition comprisinga material selected from the group consisting of sulfur trioxide and 'oleum'of S0 strength such thatSQ polymers tend tosforma n daahqnt ;materi al selected from :thegroupconsisting. of silica and titania. t

7. The method of stabilizing against S0 polymerization the material selected' from' the group consisting-10f liquid sulfur trioxide and 'oleum'of S0 strength such 'thatjSO polymers tend to form comprising incorporatingtherein a compoundselected fromrthejgroup consisting of silica and titania in an amountupto 10% by 50 content.

8. The method of stabilizing against S0 polymerization the material selected from the group consisting of Weight based on liquidsulfur trioxideand oleum ofSO strength .suchgthat polymers ;tend to form comprising incorporating therein acqmpound selected from the group consisting of -sili;ca aud titania in an amount up to 10% by weight based on S0 contentand then heating the resulting composition for several hours at a temperature, in the range of C.

to "C.

References Cited in theifile. of this patent UNITED STATES PATENTS Great Britain M21326; 1952 

1. THE METHOD OF STABILIZING AGAINST SO3 POLYMERIZATION A COMPOUND SELECTED FROM THE GROUP CONSISTING OF LIQUID SULFUR TRIOXIDE AND OLEUM OF SO3 STRENGTH SUCH THAT SO3 POLYMERS TEND TO FORM COMPRISING INCORPORATING THEREIN ABOUT 0.2 TO 5% BY WEIGHT BASED ON SO3 CONTENT OF A MATERIAL SELECTED FROM THE GROUP CONSISTING OF SILICA AND TITANIA AND THEN HEATING FOR SEVERAL HOURS AT A TEMPERATURE IN THE RANGE OF 50*C. TO 100*C. 